For many applications, the adhesion of an exterior coating to a substrate is extremely critical. Whether the coating is for electrochemical protection such as an anodic coating of flame sprayed aluminum on a steel offshore structure or to provide barrier protection or decorative enhancement such as a paint coating on a structure to be protected, it is critical that the adhesion of the coating to the substrate be within well defined limits, depending on the application, in order to provide the requisite protection desired of the coating. Any protective advantage afforded by a coating is quickly lost when the coating separates from the substrate.
In order that the tests be reliably reproducible, it is essential that an adhesion tester apply purely tensile forces to the coating which are in a direction perpendicular to the coated surface. Any nonperpendicularly-directed forces can result in tearing of the coating and an inaccurate adhesion test reading.
U.S. Pat. No. 3,821,892 describes an apparatus for testing intercoat adhesion. A dolly is bonded to the test material and a tractive force is applied to the dolly which is located between a two-legged support, the legs bearing against the coated surface. While this apparatus is adequate for testing coatings on a completely flat surface, any localized irregularities within the span of the legs or any curvature in the part being tested can cause a coating to tear. U.S. Pat. Nos. 3,628,378 and 3,577,775 disclose apparatus of similar operation.
While the above-noted apparatus do provide an investigator with useful information regarding coating adhesion, the testing process, of necessity, results in the removal of at least a portion of the tested coating. Such coating removal thus at least partially destroys the usefullness of the article to which the coating is applied because of the destructive nature of the test. For this reason, coating adhesion tests on articles to be later placed in use must be made only in a very limited number of areas and only in those areas in which the holiday in the coating which is created does not present a critical problem.